It has been demonstrated that CD8 positive cytotoxic T lymphocytes (CTLs) recognize epitope peptides derived from tumor-associated antigens (TAAs) on major histocompatibility complex (MHC) class I molecule, and then kill the tumor cells. Since the discovery of the melanoma antigen (MAGE) family as the first example of TAAs, many other TAAs have been discovered through immunological approaches (Boon T, Int J Cancer 1993 May 8, 54(2): 177-80; Boon T & van der Bruggen P, J Exp Med 1996 Mar. 1, 183(3): 725-9), and some of the TAAs are now in the process of clinical development as immunotherapeutic targets.
Identification of new TAAs, which induce potent and specific anti-tumor immune responses, warrants further development of clinical application of peptide vaccination strategies in various types of cancer (Harris C C, J Natl Cancer Inst 1996 Oct. 16, 88(20): 1442-55; Butterfield L H et al., Cancer Res 1999 Jul. 1, 59(13): 3134-42; Vissers J L et al., Cancer Res 1999 Nov. 1, 59(21): 5554-9; van der Burg S H et al., J Immunol 1996 May 1, 156(9): 3308-14; Tanaka F et al., Cancer Res 1997 Oct. 15, 57(20): 4465-8; Fujie T et al., Int J Cancer 1999 Jan. 18, 80(2): 169-72; Kikuchi M et al., Int J Cancer 1999 May 5, 81(3): 459-66; Oiso M et al., Int J Cancer 1999 May 5, 81(3): 387-94). Until now, several clinical trials using these tumor-associated antigen derived peptides have been reported. Unfortunately, only a low objective response rate could be observed in these cancer vaccine trials so far (Belli F et al., J Clin Oncol 2002 Oct. 15, 20(20): 4169-80; Coulie P G et al., Immunol Rev 2002 October, 188: 33-42; Rosenberg S A et al., Nat Med 2004 September, 10(9): 909-15).
One possible reason for this relative lack of efficacy could be the loss or down-regulated expression of human leukocyte antigen (HLA) class I molecules on tumor cells, which frequently occurs in solid tumors and severely impairs T cell-mediated anti-tumor responses (Cormier J N et al., Int J Cancer 1998 Feb. 9, 75(4): 517-24; Hicklin D J et al., Mol Med Today 1999 April, 5(4): 178-86; Paschen A et al., Int J Cancer 2003 Mar. 1, 103(6): 759-67). Even if potent cytotoxic T lymphocytes (CTLs) are induced by cancer vaccine targeting tumor associated antigens, the CTLs fail to recognize the target cells when they do not express a sufficient amount of HLA class I molecules.
Tumor angiogenesis is critically involved in the progression of tumors. It has been already demonstrated that an effective vaccine against tumor angiogenesis could be developed according to an endothelial cell-based approach, targeting vascular endothelial growth factor receptors (VEGFRs) 1 and 2, as HLA class I molecules are not down-regulated on endothelial cells (Wada S et al., Cancer Res 2005 Jun. 1, 65(11): 4939-46; Ishizaki H et al., Clin Cancer Res 2006 Oct. 1, 12(19): 5841-9). Moreover, since these therapeutic targets are tumor-independent, the depletion of vascular endothelial cells in the tumor microenvironment could be effective against a variety of malignancies. Furthermore, tumor endothelial cells are readily accessed by lymphocytes in the bloodstream, and CTLs can directly damage endothelial cells without the penetration of any other tissue type. In addition, the lysis of even a small number of endothelial cells within the tumor vasculature may result in the destruction of vessel integrity, thus leading to the inhibition of numerous tumor cells (Folkman J, Nat Med 1995 January, 1(1): 27-31). Therefore, tumor endothelial cells are a good target for cancer immunotherapy. In order to suppress tumor angiogenesis with a specific and efficient CTL response, an appropriate target needs to be selected among molecules that are related to angiogenesis.
Tumor endothelial markers (TEMs) including TEM8, have been found to be specifically elevated in tumor-associated endothelium compared with normal tissue (St Croix B et al., Science 2000 Aug. 18, 289(5482): 1197-202). The TEM8 transcript was expressed in lung and brain tumor and liver metastasis. Therapy targeting TEM8 is applicable to a wide range of tumor types. For example, WO 2005/048943 proposes the use of vaccines comprising a vector encoding the extracellular domain of TEM8 with a vaccine encoding tumor-associated antigen. However, this document fails to provide any evidence that the introduction of the TEM8-expressing vector resulted in the induction of CTLs against tumor-associated endothelium, nor does it provide any information on the position of epitopes within the TEM8 protein.